1. Field of Invention
This invention pertains generally to electronic ballasts for gas discharge lamps. More particularly, this invention relates to a novel Transformerless high frequency ballast.
2. Description of the Related Art
There are several types of conventional gas discharge lamps which have gained wide acceptance. These types include, for example, fluorescent, krypton, high intensity discharge, mercury vapor, metal halide, and sodium lamps.
Ballasts for gas discharge lamps serve several functions. Ballasts provide sufficient voltage to ignite the gas within the lamp in order to start the lamp. Ballasts provide a source for cathode heating to accelerate lamp start-up. Ballasts limit lamp current to prevent higher-pressure gases from destroying the lamp. As a last example, ballasts need to provide a unity power factor at the input side when the input is connected to utility alternating current ("ac") lines.
Typical gas discharge lamps require a high voltage to ionize the gas. After the gas is discharged, the lamp presents a negative impedance, and the lamp voltage maintains near constant. In order to provide a high voltage to ionize gases in a lamp, traditional ballasts use a transformer to boost the voltage. After the lamp is ignited, the transformer coil serves as an inductor to limit current. Traditional ballasts are also called "magnetic ballasts." These magnetic ballasts utilize iron-cores and magnet-wire coils for low frequency power systems.
Conventional electronic ballasts use a one stage LC resonant inverter to generate high frequency ac voltage for gas discharge lamps. A resonant T-type LCL circuit as shown in FIG. 1 is described in J. Funke, "Lamp Types and Circuits," in Fluorescent Lamps and Lighting, edited by W. Elenbaas et al, The MacMillan Co., New York, 1959. By applying an input voltage with a frequency near the first stage LC resonant frequency or (f=1/2.pi..sqroot.L.sub.1 C), a high voltage will be generated across the second inductor.
Recently, high frequency electronic ballasts have been used to reduce the size of magnetic components and to improve lamp efficacy. When applying high frequency lamp currents, the arc will not extinguish at the zero crossings, thus high frequency lamp currents result in higher efficacy and in flicker elimination. Conventional electronic ballasts use transformers to produce high voltage and use additional inductors to limit the lamp current. Such ballasts consume large amounts of power, and the overall lamp-ballast efficiency improvement of such ballasts is limited by their inductors.
Typical gas-discharge lamps present a constant voltage across the lamp after the lamp is ignited. Their dimming control is accomplished by varying the lamp voltage or current which requires a feedback circuit, resulting in complicated and expensive control circuitry. The voltage or current control requires a feedback circuit for pulse width modulation. Such mechanism is expensive and not suitable for a universal ballast. Because the lamp voltage must be maintained constant after the lamp is ignited, the control mechanism needs to adjust the lamp current to vary the brightness. In traditional designs, the lamp start-up voltage is typically 100 percent higher than the steady-state voltage. This high start-up voltage causes filament sputters and causes reduced lamp life.
In order to maintain gas discharge, the filament needs to be heated. This filament heating helps lamp start-up but introduces additional losses. If the voltage across the lamp is high enough, the filament voltage can be removed after the lamp is ignited. However, the dimming capability will be lost with the removal of the filament voltage. Most existing electronic ballasts provide a fixed voltage across the filament.
Prior art inverter circuits have been proposed to ignite gas discharge lamp, in which the high voltage was generated by a transformer. Because the transformer does not limit the current after arc ionization, the circuit requires additional inductor in series with the lamp. The dimming control in these ballasts requires a feedback circuit to control the inverter duty cycle.
In addition to the shortcomings of conventional ballasts, conventional ballasts suffer from other limitations. Conventional ballasts produce iron noise (electromagnetic interference) due to their magnetizing branches; they have limited lamp life due to high startup voltage; they are heavy due to their low frequency transformer; they have limited power conversion efficiency due to core and coil losses.